The present invention relates generally to a converter, and more particularly to a digital-to-analog converter. Generally speaking, an ordinary digital-to-analog converter includes sets of switches which are electrically connected to one another, a generator responsive to a digital input signal for controlling the set of switches, a set of resistors, and an output buffer. It is to be noticed that what kinds of circuit elements are chosen and how the elements are connected are the key points to the functions and effects of the digital-to-analog converter.
In the conventional digital-to-analog converter, the generator uses an up-counter or a down-counter resulting in a disadvantage that owing to the increased probability of introducing the signal glitch, the noise in the output analog signal is thus increased and the distortion is therefore increased. Taking the 4-bit up-counter for example, when the up-counter counts from 7 (with a binary representation 0111, i.e., bit4=0, bit3=bit2=bit1=1) to 8 (with a binary representation 1000, i.e., bit4=1, bit3=bit2=bit1=0), bit4 changes from 1 to 0, and bit3, bit2, and bit1 change from 0 to 1. Because of the propagation delay (the time delay), there are undesired transient states such as 0110, 0100, 0010 . . . etc. generated between 0111 and 1000. Therefore, if the seventh switch is set to be on and the other switches are set to be off when the counter counts 7 and if the eighth switch is set to be on and the other switches are set to off when the counter counts 8, some switches will be undesiredly turned on because of the undesired transient states resulting in some undesired noise being introduced to the converter output. Therefore, the output quality of the digital-to-analog converter is seriously affected.
Moreover, the conventional digital-to-analog converter chiefly uses NMOS switches or PMOS switches. Because the NMOS switch is turned on under a signal ranging from V.sub.DD (the power supply) minus V.sub.TN (the NMOS threshold voltage) to V.sub.SS (a ground voltage), the NMOS switch cannot be turned on under a signal ranging from V.sub.DD minus V.sub.TN to V.sub.DD. Similarly, the PMOS switch is turned on under a signal ranging from V.sub.TP (the PMOS threshold voltage) to V.sub.DD, so the PMOS cannot be turned on under a signal ranging from V.sub.SS to V.sub.TP. That is to say, owing to a smaller working range of PMOS or NMOS, some signals which are partly not in said range will be cut off, so the distortion is seriously caused.
And further, the equivalent resistance of every switch is an important factor affecting the quality of the output signal. When the equivelant resistance is higher, the distortion is more serious, and vice versa. According to the conventional digital-to-analog converter, the equivalent resistance of the PMOS switches or the NMOS switches will vary as the applied power supply varies, so there will be different performances of the output signal because of the different applied power supplies. Taking the NMOS switch for example, the equivalent resistance is increased when the power supply VDD is decreased, and the equivalent resistance is decreased when the power supply VDD is increased. So, the PMOS switch or the NMOS switch performs differently when the different power supply is applied.
Furthermore, the PMOS switches or the NMOS switches are electrically connected to an output buffer which is strictly selected to meet the demand of impedance matching. Because the turn-on ranges of NMOS switches and PMOS switches are both narrow and are different to each other, and because the input common mode range of an operational amplifier is to be considered, it is better that an operational amplifier using an N-channel input differential amplifier is chosen to be the output buffer when the PMOS switches are applied and an operational amplifier using a P-channel input differential amplifier is chosen to be the output buffer when the NMOS switches are applied. Thus, the choice of a buffer means is restricted.
U.S. Pat. No. 4,446,436 entitled CIRCUIT FOR GENERATING ANALOG SIGNALS discloses a digital-to-analog converter. According to FIG. 1 thereof, the node (tap) voltages of the set of resistors are connected to the output through three sets of switches. Because the equivalent resistance of the switch is an important factor determining the distortion, when the equivalant resistance is higher, the distortion resulting from the noise is thus more serious, i.e., the more sets of the switches are used, the more distortion is generated. The equivalent resistance of the three sets of switches according to FIG. 1 is in such a situation that the converter is much mare prone to suffer the distortion problem. Besides, the performance of the applied NMOS switches such as T1-T16 is prone to be affected by the different applied power supply. It is therefore attempted by the Applicant to deal with the above situation encountered by the prior art.